package org.example.basic_sort;

/**
 * 小和问题
 */
public class Code05_SmallSum {

    public static int smallSum(int[] arr) {
        if(arr == null || arr.length < 2) {
            return 0;
        }
        return process(arr,0, arr.length - 1);
    }

    public static int process(int[] arr,int L, int R) {
        if( L == R) {
            return 0;
        }
        int mid = L + ((R - L) >> 1);
       return process(arr,L,mid) +
        process(arr,mid + 1,R) +
        merge(arr,L,mid,R);

    }
    public static int merge(int[] arr,int L, int M ,int R) {
        int[] help = new int[R - L + 1];
        int i =0;
        int p1 = L;
        int p2 = M + 1;
        int res = 0;
        while(p1 <= M && p2 <= R) {
            res += arr[p1] < arr[p2] ? (R -p2 + 1) * arr[p1] : 0;
            help[i++] = arr[p1] < arr[p2] ?arr[p1++] : arr[p2++];
        }
        while(p1 <= M) {
            help[i++] = arr[p1++];
        }
        while (p2 <= R) {
            help[i++] = arr[p2++];
        }
        for(i = 0;i < help.length; i++){
            arr[L + i] = help[i];
        }
        return res;
    }

    public static void main(String[] args) {
        int[] arr = {1,3,4,2,5};

        System.out.println(smallSum(arr));
    }
}
